1. Field of the Invention
The present invention relates to apparatuses and methods for reading symbol information that optically read symbol information such as a barcode.
2. Description of Related Art
Conventionally, barcodes have widely been used as measures for automatically recognizing identification information specific to an object, while the identification information having been provided to the object. Such a barcode includes a series of black-and-white sections that are called white level elements (spaces) and black level elements (black bars). For example, in the case of a barcode attached to a commodity, a light beam is radiated to the barcode for a barcode reading apparatus such as a laser scanner, a CCD camera, and the like to scan a light beam. A photo acceptance unit receives a light beam reflected from the white level elements (spaces) and black level elements (black bars), and then the reflected light beam received is photo-electrically converted for reading a variation in the reflection coefficient of the reflected light beam as multi-valued information. Generally, the multi-valued information is binarized with a predetermined threshold and then subsequently decoded.
FIG. 10 shows an example of a conventional way of binarization. The method shown in FIG. 10 is described in Japanese Industrial Standards JIS X502; and it is specified in the method that, for example in a (scanned) reflection coefficient waveform shown in FIG. 11, a maximum reflection coefficient Rmax and a minimum reflection coefficient Rmin are obtained, and then a binarization threshold GT is calculated by means of a formula shown below:GT=(Rmax+Rmin)/2
On this occasion, the (scanned) reflection coefficient waveform is a waveform that is created by dotting the reflection coefficient along a line in a direction scanning direction) intersecting the white level elements and black level elements constituting a barcode; and it is a waveform, for example, such as what FIG. 11 shows,
In the (scanned) reflection coefficient waveform shown in FIG. 11, an element judgment is made so as to assume a part with an output level exceeding the binarization threshold GT to be a white level (a space), and meanwhile a part with an output level failing below the binarization threshold GT to be a black level element (a black bar); and then a decoding operation progresses in this way.
In reality, the (scanned) reflection coefficient waveform changes in various ways, depending on printing conditions of the barcode, resolution performance of the barcode reading optical system, fluctuation in the transfer speed of the recording medium. Especially, when the conventional calculation method of binarization threshold is applied to a (scanned) reflection coefficient waveform shown in FIG. 7, the binarization threshold GT is set at a position closer to the white levels (spaces) so that precise judgment on a white/black level unfavorably becomes impossible. In the case of the (scanned) reflection coefficient waveform shown in FIG. 7, the Rmax and Rmin are 108 and 0, respectively, to make the binarization threshold GT equal to 54, and therefore the binarization threshold is set at a position closer to the white levels (spaces), as shown in FIG. 7. Accordingly, it is likely that a range for the white levels (spaces) becomes narrower than it should actually be; meanwhile a range for the black levels (bars) becomes wider than it should actually be. As a result, the conventional method has a tendency to cause a decoding error, namely to make a reading mistake.
There have been tried various improvements for further enhancing the reading accuracy of barcode reading apparatuses by dealing with these problems (e.g., refer to Japanese Unexamined Patent Application Publication No. H08-202804 (“JP H08-202804”) and Japanese Unexamined Patent Application Publication No. 2005-165949 (“JP 2005-165949”)).
JP H08-202804 discloses a provision of a function in which an electric output signal created by photo-electrically converting a reflected light beam from a barcode is amplified with a predetermined gain and meanwhile the gain of the amplifier is suitably adjusted in accordance with a signal level difference between a black level (a bar) and a white level (a space) in a barcode reading apparatus having a binarization function of the amplified reflection output signal ((scanned) reflection coefficient waveform).
JP 2005-165949 discloses a fine adjustment of a binarization threshold in accordance with a decoding result through modification of an initial value of the binarization threshold in order for re-decoding operation, in a barcode reading apparatus in which image information of a barcode developed as multi-valued information is binarized and decoded for each pixel with the predetermined threshold.
In the barcode reading apparatus disclosed in JP H08-202804, the gain of a signal amplitude is adjusted for stabilizing the accuracy of binarization according to the deviation of average values of black levels and white levels in the scanned reflection output signal (scanned) reflection coefficient waveform). However, in this case, the fluctuation in amplitude of the white levels sometimes becomes large, depending on the condition of the scanned reflection output signal or the kind of barcode to be read so that the deviation of the average values, being an important factor, becomes inaccurate. As a result, unfortunately the accuracy of binarization becomes worsened.
Meanwhile, in the barcode reading apparatus disclosed in JP 2005-165949, after the initial value of the binarization threshold is once calculated, the binarization threshold is modified through a prescribed method and the decoding operation is repeated until the decoding operation becomes settled. Then, unfortunately the processing time becomes excessively long, depending on the condition of the input signal.